For a brushless motor to efficiently generate torque, the induced voltage phase of the drive coil needs to well match the coil current phase. However, when the induced voltage phase of the drive coil is made to match the phase of coil-applied voltage, the coil current has a phase delayed from the induced voltage phase due to an armature reaction, which undesirably reduces the torque.
To solve such a problem, coil-applied voltage needs to have a phase advanced from the induced voltage phase, for which various methods have been developed. As an example, there is known a method in which the phase of a coil current is detected or estimated to control the phase advance amount of coil-applied voltage so that the current phase of the coil matches the induced voltage phase. (Refer to PTL 1 for example.)
Such a method is ideal to generate torque efficiently; however, it requires a current sensor for detecting a coil current. Another method in which a power supply current is detected using a shunt resistance, instead of a current sensor, to estimate the coil current phase causes a large estimated error or impossible estimation of the coil current phase depending on the load on the brushless motor, its inductance, and power supply voltage to it.